MAINTAINERS: Add maintainer for samsung mfd driver

There are many samsung multifunction devices which are
s2mps11, s5m8767, s5m8763 etc.
This devices can support regulator, rtc, charger.
I will be supporting the Samsung mfd drivers, that are currently
in development and will be upstreamed shortly.

Signed-off-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>